Magnetic field control in electrolysis cells using plates and/or bars

ABSTRACT

A plurality of elongated, magnetic flux conducting structures magnetically spaced adjacent the sides of a cell for producing molten aluminum by electrolysis. The magnetically spaced, flux conducting structures are effective to shield the cell from and to substantially reduce the strength of a magnetic field component extending within the cell in a direction substantially parallel to the length dimension of the elongated structures without substantially affecting magnetic field components extending in directions other than the magnetic field component extending parallel to the elongated structures.

United States Patent 1191 Rob] et al.

- 1 1 March 6, 1973 1 1 MAGNETIC FIELD CONTROL IN ELECTROLYSIS CELLS USING PLATES AND/0R BARS [75] Inventorsi Robert V F. Rohl, Monroeyille, Wayne J. Walker, Burrell, both of [73] Assignee: Aluminum Company of. America,

Pittsburgh, Pa.

221 rhea; March 18,1971

21 Appl. No.: 125,584

52 U.S.Cl.' "204/243 M, 204/244 51 1m. 01. ..C22d 3/02 [58] Field of Search; 2041243447, 67

[5 6]- References Cited UNITED STATES PATENTS 3,617,454 11/1971 Johnson .204/2431/1 3,415,724 12/1968 Heaton et a1. ..204/243 R 3,616,317 10/1971 McLellan et a1. ..204/243 M X Johnson ..204/243 M X Primary Examiner-John 1-1. Mack Assistant Examiner--D. R. Valentine Att0rneyElroy Strickland 57 ABSTRACT A plurality of elongated, magnetic flux conducting structures magnetically spaced adjacent the sides of a cell for producing molten aluminum by electrolysis. The magnetically spaced, flux conducting structures are effective to shield the cell from and to substantially reduce the strength of a magnetic field component extending within the cell in a direction substantially parallel to the length dimension of the elongated structures' without substantially affecting magnetic field components extending in directions other than the magnetic field component extending parallel to the elongated structures.

13 Claims, 2 Drawing Figures PATENTEDHAR' 6 1913 FIG. I.

MAGNETIC FIELD CONTROL IN ELECTROLYSIS CELLS USING PLATES AND/OR BARS BACKGROUND OF THE INVENTION The present invention relates generally to electrolysis cells for producing molten aluminum from a compound of the aluminum, and particularly to structures for substantially reducing the adverse effects of magnetic fields on the molten aluminum within such cells.

In the production of aluminum in an electrolysis cell, molten aluminum is formed by the electrolysis of a fused salt bath resulting in a layer of the molten aluminum, of relatively uniform thickness, being formed on the bottom of the cell. It is known that the interaction of magnetic fields and current densities within the cell produce electromotive forces on the molten aluminum layer which results insubstantial movement of the aluminum layer within the cell. This movement causes extensive erosion of the lining. This erosion, particularly at cracks where pot holes are formed, results eventually in the shut down of the cell or cells for the purposes of repairing and/or replacing the lining. As can be appreciated, the shut down of a cell and the repair or replacement of a cell liner is a very costly process since the cell is not producing during the time of shut down, and the time, labor and materials for repairing or replacing a liner are considerable.

In addition, the movement of the molten aluminum layer necessitates an unduly large separation 'of the cell's anodes from the cell liner since the moving layer tends to assume uneven vertical displacement within the cell beneath the anodes.

The magnetic fields causing the movement of the aluminum layer originate both within the cell and externally of the cell, the major source of the externally produced flux originating from bus risers for the cell anodes and from portions of the cathode buses electrically connecting the cell to an adjacent cell. The source of internally produced magnetic flux is the cell current itself, i.e., current flowing from anode to cathode through the salt bath and the aluminum layer, and from the cathode connector bars extending into the cell liner. j 7

As a general consideration, the forces that move the metal can be reduced bydecreasing the magnetic field strengths within the cell, by decreasing the current densities in the cell and in associated conductor buses, and/or by rearranging the pattern of these variables in their relationship to the cell and the molten layer of aluminum therein. For example, a prior practice of reducing the adverse influence of magnetic fields within the cell has involved various rearrangements of the cathode and anode leads and buses in their relationship to the cell. It has been found, however, that while reducing one particular magnetic field component within the cell, other magnetic field components have been adversely affected, i.e., the strength of the other components have been increased or decreased in a manner that actually increases the velocity of the circulating aluminum. This occurs because the electromotive forces thatmove the molten metal tend to move it in complex circulation patterns so that the mere reduction or elimination of one magnetic field component may increase the problem of metal movement rather than decrease it.

o the magnetic fields produced by the main current in the cell leads and buses have saturated the shell with magnetic flux. For this reason the shell no longer functions as an effective shield. The overall thickness of the cell shell cannot be increased to provide a more effective shield because the resulting increase in its flux conducting capabilities does not influence the degree of mag netic saturation.

BRIEF SUMMARY OF THE INVENTION The present invention is directed to structures which control and reduce the influence of magnetic fields within an electrolysis cell in such a manner that the movement of a layer of molten aluminum metal within the cell is stopped or at least substantially reduced. This is accomplished by eliminating or at least substantially reducing the electromotive force on the aluminum layer caused by a single magnetic field component extending through the cell and through the molten aluminum layer without affecting the forces on the molten layer produced by magnetic field components extending in directions other than the single component. In a preferred embodiment of the invention, the vertical field component is eliminated or its strength at least substantially reduced within a cell without affecting the forces on the molten aluminum layer produced by magnetic field components extending longitudinally and transversely of the cell. As explained in greater detail 7 hereinafter, this aids in allowing the two latter forces to ofi'set each other which tends to stop the movement of the molten aluminum.

The vertical field component within the cell is eliminated or the strength thereof at least substantially reduced without affecting longitudinally and transversely extending field components by the use of a plurality of magnetically spaced and vertically extending, elongated, magnetic flux conducting members, such as plates or bars, located between the molten metal within the cell and externally located sources of the vertical component. The externally located sources of the VCTII? cal component are primarily the cathode collector bars and buses conducting current from one cell to the next cell. The collector bars extend into the liner-cathode of the cell on the upstream and downstream sides thereof and are joined to the buses which connect adjacent cells. The buses which connect adjacent cells extend along the sides and ends of the cell. With this type of conductor arrangement, the elongated plates or bars are preferably directly located on or in the sides and ends of the cell as explained in greater detail hereinafter.

The plates or bars as briefly described above, provide a simple, economical means for the shielding and controlling magnetic field within an electrolysis cell. No costly rearrangement of cathode and anode leads are required, and the thickness of the overall shell need not be increased. Any magnetically conductive metal can be used as the shielding mechanism in the present invention, for example elongated pieces of scrap iron and steel, and old, used cathode collector bars.

THE DRAWING The invention, along with its objectives and advantages, will be best understood from consideration of the following detailed description in connection with the accompanying drawing in which:

FIG. 1 is a top view of an electrolysis cell (and a partial view of adjacent cells) provided with means for controlling the strengths of magnetic field components therein in accordance with the principles of the present invention; and

FIG. 2 is an elevation view of the cell of FIG. 1, with a portion thereof in section, as viewed from the upstream side thereof.

PREFERRED EMBODIMENT OF THE INVENTION Referring now to the drawing, FIG. 1 is a plan view of a cell or pot 12 particularly suitable for producing molten aluminum from a fused salt bath, the cell being electrically connected to adjacent cells and 14 (only partially shown) respectively on the upstream and downstream sides thereof. The terms upstream and downstream refer to the direction of current flow from cell to cell, the cells employed in making aluminum usually being connected in electrical series by conductors connecting the cathode of one cell to the anodes of the next adjacent cell. Specifically, the up-- stream side is the side on which the anodes are connected to the cathodes of the adjacent cell while the downstream side is the side on which the cathode bars are connected to the anodes of the adjacent cell.

The cell 12, as shown in the drawing, has an outer steel shell 16 that is preferably magnetically conductive, an inner electrically conductive carbon liner l8 and an intermediate insulating lining 20 of suitable material such as fire brick or crushed insulating material. As shown in FIG. 2, the cell can be supported on spaced I-beams 22, and the top of cell provided with a metal deck plate 24 (FIG. 2) covering the top edges of the steel shell and carbon and insulating liners. In FIG. 1, the deck plate is not shown so that the top edges of the shell and linings are exposed.

As shown in FIG. 2 and in cell 14 of FIG. 1, anode structures 25 are supported within the cell and within an electrolyte (not shown) by, electrically conductive rods 26 suitably attached to overhead anode buses 27, only one of which is shown FIG. 1, extending between laterally disposedanode buses 28 and 29. In FIG. 1, the anodes and anode buses of the cell 12 are not shown in order to simplify the figure for purposes of explaining the structures directly related to the invention. In commercial installations, the cells 10, 12 and 14 and other cells or pots connected in the circuit of the cells, i.e., in a pot line, would have the same or similar anode (and cathode) systems.

Direct current is supplied to the cells via the anodes 25 and their associated buses, and the current flow path is completed through the cell by a cathode system comprising the molten layer of aluminum, the conductive liner 18, and horizontally disposed collector bars 30 of iron or steel embedded in the liner and extending transve'rsely of the cell as indicated in dash outline in FIG. 1.

The collector bars extend outwardly from the liner and through the shell 16 on the upstream and downstream sides of the cell as best seen in FIG. 1. The bars are suitably connected to ring bus conductors 31 and 32 which are spaced from the shell and which extend respectively to the left and right ends of the cell to join 'rnain cathode buses 33 and 34 respectively. As suggested above, it is a normal practice in commercial installations to arrange a large number of cells or pots in line (single or tandem rows), in which case the cells are electrically connected in series by the main cathode buses 33 and 34 of one cell conducting its current into the anode buses 28 and 29 of the next succeeding cell in the line.

Depending upon the density of currents and the resulting strengths of the magnetic fields within the cell and in the molten aluminum, the electromotive forces on the aluminum can be quite vigorous in the cells described herein and illustrated in FIGS. 1 and 2. Some motion in the bath which serves to stir it and to distribute additions of alumina to the cell throughout the bath during operation of the cell is desirable. However, under the high operating currents presently used in the cells of types under discussion, too vigorous agitation of the bath and particularly the molten metal layer has been found to produce an inefficient pot operation for the reasons given above.

In the operation of the cell arrangement shown in FIG. 1, magnetic flux is produced about the cathode bars 30 and the buses 31 to 34 as they conduct current from the cell. This flux exists both inside of the cell (by portions of the cathode bars 30 located outside of the shell. With current densities producing a magnetic field strength sufficient to saturate the shell, the shell is not sufficient to shield the interior of cell from the flux produced externally of the cell, i.e., the flux produced in the buses 31 to 34 and in those portions of the cathode bars 30 located outside of the cell.

The magnetic flux generated about the cathode bars 30 and the conductors 31 to 34 has a component that extends at right angles to the current flow and vertically through the layer of molten metal within the cell, cell 12 for example. This vertical field component is strongest nearest the right and left ends of the cell due to concentration of the current conductors laterally spaced therefrom and due to a functioning of Ampere s Law within the cell, i.e., within the cell, as with any current carrying conductor, the strength of the resultant magnetic field is weakest (i.e., zero) at the center thereof where the field reverses its direction.

In the plan view of cell 12 of FIG. 1, the direction of the vertical field is into the plane of the paper at the upstream, left hand end of the cell as indicated by a circled X, and out of the plane of the paper on the upstream right hand end of the cell as indicated by a circled dot. Similarly, at the right hand end of the cell, on the downstream side thereof, the vertical field component is reverse, i.e., it is into the plane of the paper. On the downstream side, left hand end of the paper, the vertical component is out of the plane of the paper. In this manner, the cell is divided essentially into four quadrants in terms of the magnetic field therein and the resultant force that the field exerts on the metal.

The vertical field component within the layer of molten metal produces a force on the metal in a direction at right angles to the component and to the direction of horizontal current flow in the molten metal, the direction of current in the metal being usually outward from cell center. This force, as generally indicated by the arrows in the cell 12 of FIG. 1, is along the sides and ends of the cell, and because of the reverse directions of the vertical field component in the four quadrants of the cell, the forces on the molten metal move the metal toward the center of the cell on the upstream and downstream sides thereof and away from the center thereof along the left and right hand ends of the cell as indicated by the arrows.

As can be appreciated, with the molten metal moving in the directions described above, the metal will tend to circulate in four basic pools within the cell, though, due to transverse and longitudinally extending magnetic field components, in combination with the vertical component, the circulation patterns tend to be quite complex. Nevertheless, if the vertical field component can be substantially eliminated within the cell, or its' strength at least considerably reduced, the circulating forces created by the vertical field component will be substantially eliminated, the effects of the longitudinal and transverse fields tending to cancel each other as explained hereinafter. In this manner, movement of the molten metal would be stopped or reduced to an insignificant amount, thereby substantially reducing the wear on the liner 18, and allowing more accurate positioning of the anodes 25 relative to the molten metal, the latter providing improved electrical power efficiency in the operation of the cell.

In accordance with the principles of the invention, the strength of the vertical field component within a cell is considerably reduced by the disposition of substantially vertically extending, elongated, magnetically conductive plates and/or bars on the sides and ends of the cell, as shown in the FIGS. 1 and 2. More particularly, on the upstream side of the cell 12 as best seen in the plan view of FIG. 1, are located four rows or layers of plates, labelled generally 36 to 39, with each layer being comprised of a plurality of plates magnetically spaced along the side of the cell. The layer of plates closest to the outside of the cell shell 16 may be physically attached thereto by a suitable means, for example, by an insulating adhesive material or by tack welds limited in number to avoid providing a continuous magnetic path along the side of cell for reasons presently to be explained. In addition, any oxide on the plates would tend to magnetically space them from the shell.

The plates in rows 37 and 39 are shown as relatively narrow plates while the plates in rows 36 and 38 are depicted as less narrow plates. This type of plate configuration and arrangement facilitates the extension of the plates to a location near the bottom of the cell between the collector bars 30, as best seen in FIG. 2. Thus, the more narrow plates 37 include relatively long plates extending substantially the full height of the cell 12, and relatively short plates extending from the top of the cell adjacent deck plate 24 down to a location immediately above the collector bars, as shown in FIG. 2.

To insure effective shielding of the cell from the vertical magnetic field generated by the collector bars 30, the gaps between the more narrow plates 37 are covered by the wider plates 36 which straddle the narrow plates, the wider plates extending vertically between the collector bars 30 and the deck 24 of the cell 12, as indicated by dash outline in FIG. 2. The wider plates may be attached to the more narrow plates by a suitable magnetically nonconductive adhesive (not shown) which functions further to magnetically space the wider plates from the narrow ones and from each other. In addition, any oxide on the wider plates would tend to insulate the wider plates magnetically from the narrow plates as well as from the shell 16 and from each other.

The order of the wider and narrower plates 36 and 37, of course, can be reversed, in which case, the wider plates would be closest to the side of the cell and the steel shell 16. Similarly, a single layer of plates having a greater thickness dimension may be employed in place of the double layer as represented by 36 and 37, in which case, the lower end of each plate could be slotted or otherwise dimensioned to accommodate a collector bar or bars 30.

The plates of the layers 38 and 39, which are visible only in FIG. 1, are embedded in the liner 18 of the cell 12, internally of the steel shell 16. Like the plates of layers 36 and 37, the plates 38 and 39 comprise less narrow plates (38) located to straddle spaces between the more narrow plates (39) to further insure the substantial reduction of the vertical field component within the cell without affecting, in any substantial manner, the other magnetic field components existing within the cell. A portion of the more narrow plates extend from the top of the cell down to and between the collector bars 30 while the less narrow plates preferably extend between the top of the cell and the collector bars. v

As seen in FIG. 1, the cathode ring buses 31 and 32 extend laterally to the main cathode buses 33 and 34 respectively. The current flow in these buses adds further to the strength of the vertical field component at the ends of cell on the upstream side thereof. To shield the cell from the field generated by these buses, elongated magnetically spaced, conductive bars 41 and 42 are located between the ends of the cell and the buses 33 and 34 respectively, the length dimension of the bars (like that of the plates 36 to 39) extending in the direction (vertical) of the field to be shielded.

In the embodiment of FIGS. 1 and 2, the shield bars 41 and 42 are conveniently supported between the cell and buses 33, 34 by having the lower portion of the bars extend into the cell liner 18, though the bars may be suitably located outside of the cell shell, and plates (not shown) may be used in place thereof or in conjunction therewith.

The vertical bars 41 and 42 extend well above the deck plate 24 and at least to a height equal to that of the end risers (where used) and anode buses for the cell being shielded. In FIG. 1, the end risers and anode buses for the cell 12 are not shown for purposes of drawing clarity, but the cell 12 has anode buses similar to those labelled 28 and 29 for the downstream cell 14. It is from the magnetic field generated by current flow in these buses that the vertical bars 41 and 42 are particularly adapted to shield the cell.

The shielding members embedded in the liner 18 of the cell 12, i.e., the plates 38 and 39, and the bars 41 and 42, can be conveniently placed in the material of the liner when the liner is being prepared in the construction of new cells, or in the repair and relining of old cells.

Because of the configuration and location of the buses adjacent the downstream side of the cell, the vertical field component produced thereby is not as strong as encountered on the upstream side of the cell. For this reason the downstream side has only externally located, magnetically spaced and conductive plates 44 and 45. Thus, the number of downstream plates are substantially less than that for the upstream side of the cell. The magnetic field produced by current flow in the end and riser buses on the downstream side of the' cell has been found to be beneficial in its effect on the movement of the molten metal. Thus, shielding bars at the ends of the cell on the downstream side are not employed.

Like the plates 36 and 37 on the upstream side of the cell, the downstream plates include less narrow plates 44 overlapping more narrow plates 45, and the spaces between the more narrow plates, to enhance the process of shielding the interior of the cell from the vertical field component produced by current flow in the downstream collector bars 30 and buses 31 and 32. The less narrow plates extend from the cell deck 24 and to a location immediatelyabove the collector bars 30 while those more narrow plates located between the collector bars extend from the deck to a location adjacent the bottom of the cell.

The elongated plates 36 to 39, 44 and 45, and the bars 41 and 42, asthus far described, are effective to conduct the vertical field component along their length dimension, thereby shielding the interior of the cell,

and the molten aluminum within the cell, from the vertical component without substantially effecting the longitudinally and transversely extending fields. In this manner, the electromotive forces produced in the molten aluminum by the vertical component are substantially eliminated without substantially affecting the forces produced by the longitudinally and transversely extending fields. The magnitude of the forces produced on the aluminum by these last two fields tends to increase toward the longitudinal and transverse center of the cell so that the aluminum tends to move from ends and sides of the cell inwardly towards the longitudinal and transverse centers thereof. For this reason, with the forces generated by the vertical field component being removed by the vertical plates and bars, the patterns of metal movement that would be produced by these inwardly directed forces are essentially opposed to each other so that their net effect is to cause no movement of the metal. It is for this reason that the longitudinal and transverse field components should not be disturbed in the process of reducing or eliminating the vertically extending magnetic field component in the cell construction of FIGS. 1 and 2. i

A convenient and economical source of shielding members in the present invention are used ferrous collector bars that have been removed from the liner of old cells, and that would otherwise be disposed of as scrap material. The shield bars 41 and 42 at the ends of the cell may be old collector bars, and such bars may be used along the sidesof the cell as well. Flat plates, however, have advantages over bars since they can be disposed more easily in overlapping layers, such as shown in FIG. 1. The plates need not, however, be new steel, for example. Scrap metal plates can be employed, the basic requirement being only that they conduct magnetic flux in the direction of their length dimension.

In addition, the shielding plates need not be located adjacentthe center of upstream and downstream sides of the cell since, as explained above, the vertical field at the center of the cell is zero.

From the foregoing description, it should now be apparent that a new and useful structure has been disclosed for shielding the interior of an electrolysis cell from the adverse effects of a single magnetic field component extending in a predetermined direction without substantially effecting magnetic field components extending in other directions within the cell. For example, by the effective elimination of the vertical field component only, the movement of molten metal within the cell is considerably reduced if not entirely stopped, the horizontal and transverse field components moving the metal in patterns which tend to cancel each other. With the effective eliminationof metal movement, the life of the lining of cells is substantially increased thereby reducing considerably the cost of repairing and relining cells.

The invention, though particularly advantageous in production of aluminum, is also useful in the production of other light weight metals by electrolysis, such as magnesium.

While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described our invention and certain embodiments thereof, we claim:

1. In combination with a cell for producing a light weight metal by electrolysis, with a layer of molten light weight metalbeing formed in said cell, at least one electrical conductor being spaced from but extending along at least a portion of said cell externally thereof, and being electrically connected to a plurality of collector bars extending into a cathode liner of said cell, at least a portion of said conductor when conducting currentbeing a source of magnetic flux capable of producing a magnetic field component extending in a predetermined direction through said layer of said molten metal,

a plurality of elongated, magnetic flux conducting members separate from said conductor and collector bars, and located between said layer of metal and said conductor, said elongated members extending in the direction of said magnetic fieldcomponent, and being effective to shield said layer of metal from said source of magnetic flux and thereby substantially reduce the strength of said magnetic field component within saidlayer of metal, said shielding members being magnetically discontinuous in directions other than the direction of said magnetic field component.

2. In combination with a cell for producing aluminum by electrolysis, with a layer of molten aluminum being formed, said cell having an outer shell of magnetically conductive material and being electrically connected to a similar adjacent cell by conductors extending therebetween, said conductors being spaced laterally from and extending along at least one side of said cell externally thereof, and being electrically connected to a plurality of collector bars extending into a cathode liner of said cell, at least a portion of said conductors when conducting cell current being a source of magnetic flux capable of saturating the cell shell and thereby producing a magnetic field component extending in a predetermined direction through said layer of aluminum,

a plurality of elongated, magnetic flux conducting members separate from said conductor and collector bars, and located on at least oneside of said cell between said layer of aluminum and said conductors, said elongated members extending in the direction of said magnetic field component, and being effective to shield said layer of aluminum from said source of magnetic flux and thereby substantially reduce the strength of said magnetic field component within said layer of aluminum, said shielding members being magnetically discontinuous in directions other than the direction of said magnetic field component. 7

3. The structure of claim 2 in which the flux conducting members include plate-like structures located adjacent the cell shell on the outside of the cell.

4. The structure of claim 2 in which the flux conducting members are bars located adjacent the shell of the cell. 1

5. The structure of claim 2 in which the flux conducting members include overlapping layers of plates spaced apart along at least a portion of at least one side of the cell, said layers being magnetically separated from each other.

6. The structure of claim 2 in which the flux-conducting members include plate-like structures extending into said cathode liner.

7. The structure of claim 2, in which the flux conducting members at the ends of cell include bar structures extending into said cathode liner.

8. In combination with a cell for producing aluminum by electrolysis of an aluminum compound dissolved in a fused salt bath, with a layer of molten aluminum being formed, said cell having an outer shell of magnetically conductive material and being electrically connected to a similar adjacent cell by conductors extending therebetween, said conductors including cathode conductors spaced laterally from and extending along at least one side of said cell externally thereof, at least a portion of said conductors when conducting cell current being a source of magnetic flux capable of saturating the cell shell and thereby producing a magnetic field component extending substantially vertically through said layer of aluminum,

a plurality of elongated, substantially vertically extending, magnetic flux conducting members located on at least one side of said cell between said layer of aluminum and said cathode conductors, said elongated members being effective to shield said layer of aluminum from said source of magnetic flux and thereby substantially reduce the strength of said magnetic field component within said layer of aluminum, said shielding members being magnetically discontinuous in directions other than the direction of said magnetic field component. a 9. The structure of claim 8 in which the cathode conductors include conductors spaced laterally from and extending along the ends of the cell, and a second plurality of elongated, substantially vertically extending magnetic flux conducting members located at the ends of the cell and between the layer of molten aluminum therein and said conductors extending along the ends of the cell, said second plurality of flux conducting members being magnetically spaced in directions other than the direction of the magnetic field component.

10. The structure of claim 8 in which the flux conducting members include overlapping layers of plates longitudinally spaced apart along at least a portion of at least one side of the cell, said layers being magnetically separated from each other by insulation.

11. The structure of claim 8 in which the flux conducting members include plate-like structures located adjacent the cell shell on the outside of the cell.

12. The structure of claim 8 in which the cell includes a liner structure inside of the cell shell, and the flux conducting members include plate-like structures extending into said liner structure.

13. The structure of claim 9, in which the cell includes a liner, and the flux conducting members at the ends of cell include bar structures extending into said liner. 

1. In combination with a cell for producing a light weight metal by electrolysis, with a layer of molten light weight metal being formed in said cell, at least one electrical conductor being spaced from but extending along at least a portion of said cell externally thereof, and being electrically connected to a plurality of collector bars extending into a cathode liner of said cell, at least a portion of said conductor when conducting current being a source of magnetic flux capable of producing a magnetic field component extending in a predetermined direction through said layer of said molten metal, a plurality of elongated, magnetic flux conducting members separate from said conductor and collector bars, and located between said layer of metal and said conductor, said elongated members extending in the direction of said magnetic field component, and being effective to shield said layer of metal from said source of magnetic flux and thereby substantially reduce the strength of said magnetic field component within said layer of metal, said shielding members being magnetically discontinuous in directions other than the direction of said magnetic field component.
 2. In combination with a cell for producing aluminum by electrolysis, with a layer of molten aluminum being formed, said cell having an outer shell of magnetically conductive material and being electrically connected to a similar adjaCent cell by conductors extending therebetween, said conductors being spaced laterally from and extending along at least one side of said cell externally thereof, and being electrically connected to a plurality of collector bars extending into a cathode liner of said cell, at least a portion of said conductors when conducting cell current being a source of magnetic flux capable of saturating the cell shell and thereby producing a magnetic field component extending in a predetermined direction through said layer of aluminum, a plurality of elongated, magnetic flux conducting members separate from said conductor and collector bars, and located on at least one side of said cell between said layer of aluminum and said conductors, said elongated members extending in the direction of said magnetic field component, and being effective to shield said layer of aluminum from said source of magnetic flux and thereby substantially reduce the strength of said magnetic field component within said layer of aluminum, said shielding members being magnetically discontinuous in directions other than the direction of said magnetic field component.
 3. The structure of claim 2 in which the flux conducting members include plate-like structures located adjacent the cell shell on the outside of the cell.
 4. The structure of claim 2 in which the flux conducting members are bars located adjacent the shell of the cell.
 5. The structure of claim 2 in which the flux conducting members include overlapping layers of plates spaced apart along at least a portion of at least one side of the cell, said layers being magnetically separated from each other.
 6. The structure of claim 2 in which the flux conducting members include plate-like structures extending into said cathode liner.
 7. The structure of claim 2, in which the flux conducting members at the ends of cell include bar structures extending into said cathode liner.
 8. In combination with a cell for producing aluminum by electrolysis of an aluminum compound dissolved in a fused salt bath, with a layer of molten aluminum being formed, said cell having an outer shell of magnetically conductive material and being electrically connected to a similar adjacent cell by conductors extending therebetween, said conductors including cathode conductors spaced laterally from and extending along at least one side of said cell externally thereof, at least a portion of said conductors when conducting cell current being a source of magnetic flux capable of saturating the cell shell and thereby producing a magnetic field component extending substantially vertically through said layer of aluminum, a plurality of elongated, substantially vertically extending, magnetic flux conducting members located on at least one side of said cell between said layer of aluminum and said cathode conductors, said elongated members being effective to shield said layer of aluminum from said source of magnetic flux and thereby substantially reduce the strength of said magnetic field component within said layer of aluminum, said shielding members being magnetically discontinuous in directions other than the direction of said magnetic field component.
 9. The structure of claim 8 in which the cathode conductors include conductors spaced laterally from and extending along the ends of the cell, and a second plurality of elongated, substantially vertically extending magnetic flux conducting members located at the ends of the cell and between the layer of molten aluminum therein and said conductors extending along the ends of the cell, said second plurality of flux conducting members being magnetically spaced in directions other than the direction of the magnetic field component.
 10. The structure of claim 8 in which the flux conducting members include overlapping layers of plates longitudinally spaced apart along at least a portion of at least one side of the cell, said layers being magnetically separated from each other by insulation.
 11. The structure oF claim 8 in which the flux conducting members include plate-like structures located adjacent the cell shell on the outside of the cell.
 12. The structure of claim 8 in which the cell includes a liner structure inside of the cell shell, and the flux conducting members include plate-like structures extending into said liner structure. 